The hunt for Planet Nine reveals some strange, far-out objects

Astronomers hunting for Planet Nine have found some 50 new objects lying beyond the orbit of Neptune – including a few that may help them track down this elusive, distant world.

The new discoveries, described in a paper accepted to the Astronomical Journal, mark another step toward finding this mysterious giant planet, which could help rewrite our understanding of our own solar system.

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In 2014, astronomers Scott Sheppard of the Carnegie Institution for Science and Chadwick Trujillo of Northern Arizona University announced that they'd found an object with the most distant orbit ever observed in the solar system – a possible dwarf planet called 2012 VP113 that lay far beyond Pluto.

2012 VP113 (nicknamed "Biden") was a baffling object, because its orbit placed it in what was thought to be a no man's land between the Kuiper belt (the icy ring of distant debris whose largest member is Pluto) and the Oort cloud, a far-out, spherical shell of debris that scientists think surrounds the solar system.

The Kuiper belt lies past Neptune's orbit from 30 to 55 astronomical units; the Oort cloud stretches from about 5,000 to 100,000 astronomical units from the sun. (One astronomical unit is the Earth-sun distance, or roughly 93 million miles.) Nothing significant was really thought to hang out in the thick doughnut of space between those two, until the 2003 discovery of a roughly 600-mile-wide rock called Sedna, whose closest point to the sun (or perihelion) lies 76 astronomical units away. 2012 VP113's perihelion is similar, falling at about 80 astronomical units.

The scientists noticed something else strange: 2012 VP113 and Sedna both seemed to be making their closest approach to the sun at similar angles. This could mean that something big, far out in the darkness, was tugging on both their orbits in the same way.

Then, early this year, a pair of Caltech scientists (Mike Brown and Konstantin Batygin) announced that they'd found evidence for a planet that was roughly 10 times as massive as Earth, based on their analysis of the orbits of several distant objects. With a perihelion of 200 astronomical units, this mysterious giant could take 10,000 years to orbit the sun.

Since then, both teams have been scanning the inky heavens looking for signs of other distant objects whose motions may reveal more information about this mysterious planetary presence. In this new paper, Sheppard and Trujillo say they've found some 50 previously unknown objects, the vast majority of which have orbits too close to Neptune to be useful.

One of the remaining objects, 2014 SR349, does seem to have the right orbital qualities to be one of these extreme trans-Neptunian objects affected by Planet Nine. Another one, 2013 FT28, has a strange mix of characteristics that could mean its orbit still responds too much to Neptune's tug to be helpful in the search.

The researchers also discovered 2014 FE72, which has such an elliptical path (stretching all the way out to roughly 4,000 astronomical units) that they think it's the first outer Oort cloud object whose orbit lies entirely beyond Neptune. This means 2014 FE72's orbit could have been influenced by outside forces, such as the galactic tide and distant stars. It's also possible that Planet Nine could have left its mark on this space rock's path.

The population of extreme trans-Neptunian objects is sparse; only about five with orbits useful for finding Planet Nine have been found so far. But once they double that number, Sheppard said, scientists should be able to pin down with much higher certainty whether Planet Nine is really out there.

"I think we're maybe about 80% sure there's something else out there," Sheppard said, "but I think we still need to find more of these smaller objects that can lead us to the bigger object."

Finding Planet Nine (or Planet X, as some are calling it) could alter our understanding of the dynamics of the early solar system, Sheppard said. Even though the known planets now lie in stable orbits, that wasn't always the case, particularly in the case of the giant planets in the outer solar system. If Planet Nine formed close to Jupiter or Saturn, those massive bodies' gravitational influence could have ended up hurling their much smaller sibling farther into the void.

"I like to call it the runt of the giant planet family," Sheppard said. "It got pushed out of the way and that's why it's not as big as Jupiter or Saturn."

As scientists have discovered planets around other stars, they've noticed that many of them are not in very stable orbits. What happened to allow the gas giants in our own planet to settle down into their calm, circular paths? Planet Nine might offer some clues.

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It's a question that also has implications for the emergence of life, because a gas giant with an unstable orbit would wreak havoc with a smaller, rocky, habitable-zone planet.

"If that were to happen in our solar system, life as you know it would not exist on Earth," Sheppard said.

I like to call it the runt of the giant planet family.

Scott Sheppard

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It's also possible that Planet Nine was a planet outside the solar system that was captured by the sun's gravitational tug. If that's the case, it could tell us more about whether the sun had closer interstellar neighbors early in its history.

The search has the scientists itching to find more of these objects, which have turned out to be a little sparser than they anticipated. It certainly doesn't help that these space rocks are incredibly difficult to find, given how little of the sun's light reaches them, and how little of that paltry illumination bounces all the way back to Earth. You need a powerful telescope that also has a large field of view – and since those are in high demand, it's hard to get a lot of observation time on them.

"We thought we'd find a few more, a little faster," Sheppard said.

Still, the two teams of scientists are on the hunt, each racing to find the next clue to the existence, and nature, of Planet Nine. But they're exchanging information to make sure they cover as much of the interesting sections of sky as possible, Sheppard added.

"We don't want to turn over the same rock twice, so we are kind of sharing our field and making sure we're not doing the same thing," he said.